Method for fine-line interferometric lithography

ABSTRACT

In microelectronic processing, the method of producing complex, two-dimensional patterns on a photosensitive layer with dimensions in the extreme submicron range. A photosensitive layer is first exposed to two beams of coherent radiation to form an image of a first interference pattern on the surface of the layer. The layer is subsequently exposed to one or more interference pattern(s) that differ from the first interference pattern in some way, such as by varying the incident angle of the beams, the optical intensity, the periodicity, rotational orientation, translational position, by using complex amplitude or phase masks in one or both of the coherent beams, or a combination of the above. Desired regions of the complex pattern thus produced are isolated with a further exposure of the photosensitive layer using any conventional lithography.

.Iadd.The U.S. Government has a paid-up license in this invention andthe right in limited circumstances to require the patent owner tolicense others on reasonable terms as provided for by the terms ofContract No. 502-MC-91 with Semantech and Contract No. AFOSRF490-89-C-0028 with the Air Force Office of Scientific Research.

This application is a reissue of Ser. No. 07/945,776, now U.S. Pat. No.5,415,835..Iaddend.

FIELD OF INVENTION

This invention relates to microelectronic circuits and more particularlyto the use of interferometric patterning in optical lithography toproduce complex, high density integrated circuit structures.

BACKGROUND OF THE INVENTION

The miniaturization of integrated circuits has been underway ever sincethe first demonstration of an integrated circuit Using dynamic randomaccess memory (DRAM) as a benchmark, current expectations of devicegenerations, dates of peak production, and lithography criticaldimension are: (4 Mb, 1994, 0.8 μm); (16 Mb, 1997, 0.5 μm); (64 Mb,1999, 0.35 μμ); (256 Mb, 2003, 0.25 μm) and (1 Gb, 2006, 0.15 μm)projections from R. J. Kopp, Semiconductor International 15, 34-41(1992)!. In the industry news section in the same issue of this trademagazine (page 11), there is a report of a MICROTECH 2000 workshopcosponsored by the National Advisory Committee on Semiconductors (NACS)and the Office of Science and Technology Policy (OSTP). The reportedrecommendation relative to lithography is: "An experimental lithographycapability that can print features of 0.10 to 0.15 μm will be requiredby 1994 in sufficient volumes to allow essential process andmanufacturing equipment development This need may require newelectron-beam mask or direct wafer writing tools, or a capability inadvanced X-Ray or phase-shift optical lithography. Research anddevelopment for several lithography alternatives will have to besupported for the next several few years to determine what system isbest suited for production."

Imaging optical lithography, in which a mask image is projected onto aphotoresist layer on the wafer, dominates today's manufacturing. Twoequations describing the optical diffraction of the optical systemdetermine the characteristics of the image. The minimum resolution, r,is proportional to the lens numerical aperture, or

    r˜1/NA

and the depth of focus (DOF)

    DOF˜1/(NA).sup.2

where 1 is the wavelength and NA the lens numerical aperture. Thesesimple equations point out some of the difficulties in extending opticallithography to the extreme submicrometer regime, ie., about 0.1 μm or100 nm. Refractive optics are available only up to approximately 200 nmat shorter wavelengths almost all materials become strongly absorptiveand unusable. There are several efforts underway to use reflectiveoptics at short wavelengths. However, there remain significant materialsproblems, particularly at X-Ray wavelengths and the NAs of these systemsare significantly lower than for refractive systems, giving away some ofthe wavelength advantage for imaging small areas.

Considerable interest and attention have been given to new X-ray lensesbased on grazing incidence filamentary propagation through hollow"waveguides." This remains a difficult problem without a demonstrationof a high-efficiency, high numerical aperture, manufacturable lens witha field-of-view that can accommodate today's growing field sizes. Fromthe experience of longer wavelength optical lithography using refractivelenses, the optical train can easily be the most complex and expensivepart of a lithography tool.

The progression to short wavelengths to improve the minimum resolutioncarries a concomitant penalty in the reduction of the depth-of-focus(DOF). This has motivated efforts at multilayer resists with strongabsorption layers, as well as efforts at improved planarization ofcircuits to eliminate topographic variations that would cause differentparts of the circuit to image at different heights. This small DOF is amajor concern for submicrometer lithography.

Briefly, major issues facing extension of conventional lithography tothe extreme submicron regime (0.1 μm) include: source technology (issuesare uniformity, spectral bandwidth, repeatability, reliability, etc.);the imaging system (again refractive optics become impossible below ˜200nm and reflective optics have inherently smaller numerical apertures);the mask technology (there are significant issues related to vibration,heating and distortion in X-Ray masks which must be fabricated onpellicle substrates because of the strong X-Ray absorption of mostmaterials); and the resist technology.

For many years periodic line and space gratings in the extreme submicronrange have been fabricated by use of two interfering coherent beams. Fortwo beams incident at angles θ and -θ to the surface normal, the periodof the interference pattern is λ(2 sin θ). For readily availablewavelengths (361-nm Ar-ion laser) and angles (θ˜75° ) this gives aperiod as small as 187 nm. The resulting grating pattern is a periodicline and space array; the critical dimensions of the lines areadjustable using nonlinearities in the expose and develop processes toroughly 1/3 of this dimension or 60 nm.

SUMMARY OF THE INVENTION

The present invention provides complex, two-dimensional patterns inintegrated circuits through the use of multiple grating exposures on thesame or different photoresist layers and the use of complex amplitudeand phase masks in one or both of the beams of illuminating coherentradiation. ("Complex, two-dimensional patterns" as used herein means apattern of multiple, interconnected and/or unconnected straight orcurved lines or bodies spaced apart from each other. "Extreme submicronrange" means distances of the order of 0.1 μm or 100 nm or less betweenlines.) Interferometric lithography may be combined with conventionallithography for the production of extreme submicrometer structures andthe flexible interconnect technology necessary to produce usefulstructures. Generally, a critical dimension (CD) of the order of 60 nmwith a pitch of 187 nm is obtainable through the process of theinvention. Although with the use of a KrF excimer laser at 248 nm, apitch of 124 nm and a CD of 41 nm can be attained. Further extension toa ArF excimer laser at 193 nm will proportionately reduce thesedimensions. In general, as laser technology continues to evolve andresults in shorter wavelength coherent sources, this technique can beadapted to produce still smaller structures.

In view of the close tolerances involved in producing patterns andmicroelectronic integrated circuits in accordance with the presentinvention, accurate alignment and position sensing is important. In thatconnection, the arrangements shown and described in Brueck et al, U.S.Pat. No. 4,987,461, and in Brueck et al. U.S. patent application Ser.No. 07/599,949, filed on Oct. 10, 1990, now U.S. Pat No. 5,343,292, maybe used to particular advantage.

The photoresist exposure process of the invention takes advantage of thefact that, in terms of dimensional thicknesses of photoresists(typically 1-2 μm), there are no DOF limitations for the two interferingcoherent optical beams. That is, for two interfering plane waves, thereis no z or depth dependence of the pattern in the direction bisectingtheir propagation directions. For coherent optical beams, the depth offield dependence is set usually by the shorter of the beam cofocalparameter or, less usually, the laser coherence length. For larger laserspots the confocal parameter is many centimeters. For typical cw lasers,i.e. Ar-ion lasers at 361 nm, the laser coherence length is on the orderof meters. The DOF of interferometric lithography is essentiallyunlimited on the micrometer scale of the thin-films employed insemiconductor manufacturing.

Another feature of the process in accordance with the invention involvesthe provision of large dimensions over which a sub-micron structure maybe fabricated. Interferometrically defined gratings have long beenavailable with dimensions up to 5×25 cm² or larger, approximately afactor of 10 larger in linear dimension than the typical field sizes oftoday's integrated circuits. Further, this can be achieved atultraviolet wavelengths for which photoresist is already well developed.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference is now made to the drawings in which like reference numeralsrefer to like parts and in which

FIGS. 1 and 2 are diagrammatic views of alternative versions ofapparatus employed to carry out the process of the invention;

FIG. 3 is a scanning electron microscopic (SEM) view of an exposed anddeveloped latent image in a photoresist from which patterns may beformed in a semiconductor wafer;

FIGS. 4-7 are SEM views of different complex two-dimensional patternsproduced from the developed photoresist image in FIG. 3 in asemiconductor wafer depending on the kind of transfer process used;

FIGS. 8-14 are SEM views of other complex two-dimensional patternsfabricated in semiconductive material in accordance with the invention;

FIG. 15 is a view of a cross section of a phase-amplitude mask inaccordance with an embodiment of the invention;

FIGS. 16-19 are schematic views of exposure stages illustrating theprocess of making an interdigitated or interleaved structure inaccordance with an embodiment of the invention;

FIG. 20 is a SEM view of an interdigitated structure produced by themethod outlined, and

FIG. 21 is an illustration of an embodiment of the invention when usedin combination with conventional imaging lithography to produce asingle, isolated line constituting the pattern.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIGS. 1 and 2, a wafer 11 having a photosensitive layer 13and substrate 14 is positioned on a movable table 15. The table 15 issupported on a shaft 17 and is arranged to be rotated and translated intwo-dimensions respectively via controls 19 and 21 which controlmechanical rotational and translational motion producing motors andlinkages generally indicated by the numeral 23. The motors and linkages23 and controls 19 and 21 need not be shown in detail since they arewell known in the art and may be of any suitable well knownconstruction.

Coherent optical beams 25 and 27 provided by any suitable well knownsource or sources are directed at a variable angle A from the verticalor system axis 29 toward each other and toward the photoresist layer 13to form an interference pattern on the photosensitive layer 13. Thearrangement shown in FIG. 2 is identical to that of FIG. 1 with theaddition of a phase-amplitude mask 31 in the path of beam 25 or aphase-amplitude mask 33 in the path of both beams 25 and 27 in theirinterference region at the surface of the photosensitive layer 13, orboth The beams 25 and 27 of coherent radiation may be lasers and may beprovided in any suitable well known manner so that they are from thesame source and are essentially equal in intensity at the wafer whichassures a high contrast exposure.

In accordance with the invention the complex interference patternproduced on the photoresist layer or layers is varied by (a) rotatingthe wafer, (b) translating the wafer, (c) both rotating and translatingthe wafer, (d) changing the angle A, (e) varying the number ofexposures, (f) varying the optical intensity. (g) using aphase/amplitude mask in one or both illuminating beams of coherentradiation, or (h) employing any combination of (a)-(g). Furtherflexibility is offered by a combination of any of (a)-(g) withconventional or imaging lithography techniques as are well known.

As an alternative method, first a single or multiple set ofinterferometric exposures are carried out in photosensitive layer. Thesubsequent pattern is then developed and transferred to a semiconductorsubstrate by any of the well known commercially available techniques.This substrate is then again recoated with a photoresistive layer, andsingle or multiple exposure processes can be repeated with the aid ofthe alignment position sensing arrangement described in Brueck, et al.in U.S. Pat No. 4,987,461.

Referring to FIG. 3, the image depicted is a rectilinear array ofcircular dots on the photosensitive layer about 300 nm apart from eachother in the x and y axes. The photoresist layer is developed andtransferred into the Si sample by a plasma-etch process. The interiorsof the circles are etched into the Si. A potentially very large scaleapplication of structures such as this is in the fabrication offield-emission flat panel displays which require large fields (up tolarge-screen television size or greater) of submicrometer field emittertips. This lithography is preferably be carried out on glass plateswhich are much less polished than today's Si wafers.

In accordance with the invention, the image of FIG. 3 is produced on thephotosensitive layer 13 by two exposures of the layer to an interferencepattern produced by the two coherent optical beams 25 and 27 as follows:With a wavelength of 488 nm for each laser beam 25 and 27 and angle A=50degrees, the photosensitive layer is subjected to a first exposure withthe period of the interference pattern being 0.3 microns and a secondexposure with the same period and other parameters but with the waferrotated 90 degrees about the axis 29. The length of each exposuredepends upon the nature of the photoresist and the optical wavelengthand intensity and, as an illustration, for a photoresist comprising KTI1350, is about 60 seconds.

The pattern shown in FIG. 3 may then be transformed into the gratingstructure shown in FIGS. 4-7 by any of several well known processes asfollows: FIG. 4--by plasma etching into silicon; FIG. 5--by reactive ionetching into GaAs; FIG. 6--by wet chemical etching into silicon, andFIG. 7--by ion beam milling into silicon.

Turning to FIGS. 8-14, the patterns shown therein are produced by plasmaetching from photoresists having complex images thereon produced by theimaging scheme of the present invention as indicated in the followingtable 1:

                  TABLE 1    ______________________________________                     Period     Rotation                                        Beam Angle    FIG     Exposure (mm)       (deg)   (deg)    ______________________________________    8       First    1.0 micron 0-deg   14-deg            Second   2.0        0-deg   7-deg            Third    1.0        90-deg  14-deg            Fourth   2.0        90-deg  7-deg    9       First    1.0 micron 0-deg   14-deg            Second   1.5        0-deg   9.4-deg            Third    1.0        90-deg  14-deg            Fourth   1.5        90-deg  9.4-deg    10      First    0.6 micron 0-deg   24-deg            Second   0.7        0-deg   20.4-deg            Third    0.8        0-deg   17.8-deg            Fourth   0.6        90-deg  24-deg            Fifth    0.7        90-deg  20.4-deg            Sixth    0.8        90-deg  17.8-deg    11      First    0.95 micron                                0-deg   15-deg            Second   1.0        0-deg   14-deg            Third    0.95       90-deg  15-deg            Fourth   1.0        85-deg  14-deg    12      First    0.95       0-deg   15-deg            Second   1.0        5-deg   14-deg            Third    0.95       90-deg  15-deg            Fourth   1.0        85-deg  14-deg    13      First    0.95 micron                                0-deg   15-deg            Second   1.0        5-deg   14-deg            Third    0.95       90-deg  15-deg            Fourth   1.0        90-deg  14-deg    14      First    0.95 micron                                0-deg   15-deg            Second   1.0        0-deg0  14-deg            Third    0.95       90-deg  15-deg            Fourth   1.0        90-deg  14-deg    ______________________________________

It is clear from the images produced in connection with FIGS. 3 thru 14that, in accordance with the invention, many other complex patterns maybe produced in the manner described. The examples discussed inconnection with FIGS. 3-14 by no means exhaust the rich array ofpossibilities of patterns including those required for highly repetitiveintegrated circuit elements such as DRAMs Of course, for theseapplications an aperiodic wiring pattern must ultimately be superimposedon this structure in any suitable well-known manner. For DRAMS even theinterconnection patter is highly regular since these circuits areusually addressed in a matrix fashion. Only at the periphery of the DRAMregion do highly aperiodic patterns occur.

It is understood that in accordance with the invention a very wide rangeof structures that can be fabricated. The aerial image for each exposureis simply a sine function:

    I(x)=A{1+sin (qx+φ)}                                   (1)

where the amplitude A, period 2p/q and phase f are set by the incidentoptical beams. Nolinearities in the exposure, develop and etch processesresult in a higher-order terms in a Fourier series expansion at the sameperiod and phase as the original image. That is:

    S(x)=ΣA.sub.n sin (nqx+φ)                        (2)

where S(x) is the resulting pattern on the wafer and the coefficientsA_(n) are the result of these nonlinear processes. Most often, the A_(n)will be a monotonically decreasing function of n. Finally, with multipleexposures the result for the pattern is:

    S(r)=ΣΣA.sub.nm sin (nq.sub.m ·r+φ.sub.m).(3)

This is a two-dimensional Fourier transform, and thus, in accordancewith the invention, any pattern definable by the transform can besynthesized. As a practical matter, this is restricted in the range of|9m| to 4π/2 and, of course, there is no independent control of eachA_(nm). Nevertheless, the transform provides the basic rule giving riseto the very large variety of patterns that may be realized throughpatterning in accordance with the principles of the invention.

Additional flexibility in pattern generation may be introduced throughthe use of amplitude and/or phase masks for one or both of the exposurebeams. Phase/amplitude masks may take any desired form depending on thedesired pattern. The mask 41 shown in FIG. 15 has two thickness-varied(i.e., path length-varied on the scale of the frequency of the coherentbeam radiation), phase modification sections 43 and 45 and two amplitudeor shadow or stenciled sections 47 and 49. Of course, a mask need onlyhave phase or amplitude portions or both.

An example of patterning employing a mask is provided in the highlyuseful, interleaved or interdigitated structure shown in the embodimentof the invention of FIGS. 16-19. The end result pattern shown in FIG. 19is produced by first exposing a 1-μm pitch grating over the entire areaof the photoresist to produce the exposed photoresist image patternshown in FIG. 167. Next, two sequential exposures are made through asimple shadow mask (e.g., a mask such as is shown in FIG. 15 with eithershadow portion 47 or 49) at twice the pitch (2 μm) over the top andbottom halves of the wafer as shown in FIGS. 17 and 18.

The wafer is then translated by 1 μm between these two later exposuresso that alternate lines of the original grating are eliminated above andbelow the pattern to produce the pattern shown in FIG. 19.

The structure shown in the SEM of FIG. 20 was fabricated by theforegoing process. The following Table II shows the steps taken toproduce the image shown in FIG. 20. In this case, the image in thephotosensitive layer 13 is essentially the same as the image produced byplasma etching, and in producing the image, the wafer was not rotatedabout axis 29 and instead was translated and apertures were located inthe position 33 as shown in FIG. 2.

                  TABLE II    ______________________________________                        Beam                    Period   Translation                                    Angle  Aperture    FIG No.           Exp      (μm)  (μm)                                    (deg.) location    ______________________________________    20     first    1.0      0.0    14     none           second   2.0      0.0    7      top           third    2.0      1.0    7      bottom    ______________________________________

Such an interdigitated structure with submicrometer spaces of about 100nm between the fingers has application. for example, as a large areasubmicrometer particle detector by fabricating an interdigitated metalgrid structure and monitoring the conductivity induced by small numbersof particles shorting out the fingers.

Except for the arrangement of the present invention, no other techniqueexists that can be used to economically fabricate these interleavedstructures over very large areas with extreme sub-micrometer dimensions.These structures are also useful for high-speed optical detectors wherethe transit times across the sub-micrometer gap determines the detectorspeed. Indeed, this interdigitated structure is commonly used for a widearray of sensors. The capability provided by interferometric lithographyof the present invention will enhance the functionality of many of thesedevices.

Reference is now made to FIG. 21 which shows an embodiment of theinvention used in combination with conventional lithography. In general,the combining of the interferometric lithography of the presentinvention with conventional imaging lithography adds other possibilitiesto the structures that may be fabricated. As one example, FIG. 21illustrates the fabrication of an isolated line with a submicrometercritical dimension (CD) using a relatively coarse pitch (say 1-2 μm)grating structure and isolating a single line with a box defined byconventional lithography. Specifically, as shown in the figure, agrating 51 is exposed on the photosensitive layer using a 1 μm pitch.The next exposure is made via a mask to provide a 1.5 μm wide box 53which masks out the other lines of the grating. The end result is thedesired single line 55 which will result after appropriate fabricationsuch as plasma etching.

Single lines have immediate use, for example, as the gate structure inhigh-speed field-effect transistors (FET). Commercial devices currentlyhave gate dimensions of ˜0.25 μm, fabricated by c-beam lithography.Laboratory research devices have been made with gates as small as 5 nmusing focused ion-beam lithography. Both of these are serial processesin which each gate must be written sequentially resulting in lowthroughput and yield. The present invention offers the possibility ofparallel writing of submicrometer gates throughout a large field of viewcircuit or set of circuits, very much as integrated circuits areconventionally fabricated. This will result in dramatically reducedmanufacturing cost and improved yield.

We claim:
 1. In microelectronic processing, the method of producing atwo-dimensional complex pattern on a photosensitive layer said patterncontaining structures with dimensions in the extreme submicron range,comprising the steps of:a) exposing the photosensitive layer for a firsttime to two beams of coherent radiation which form an image of a firstinterference pattern on the surface of said layer; b) exposing thephotosensitive layer for at least one subsequent time to two beams ofcoherent radiation which form an image of at least one subsequentinterference pattern, such that said subsequent interference pattern orpatterns referenced to the photosensitive layer are each different fromthe first pattern; c) isolating desired regions of said complex patternwith a further exposure of the photosensitive layer using anyconventional lithography.
 2. The method of claim 1 wherein thephotosensitive layer is rotated between exposures such that eachsubsequent interference pattern differs in rotational orientationrelative to said first interference pattern.
 3. The method of claim 1wherein the photosensitive layer is translated between exposures suchthat each subsequent interfere pattern is offset from said firstinterference pattern.
 4. The method of claim 1 wherein thephotosensitive layer is both rotated and translated between exposuressuch that each subsequent interference pattern different from said firstinterference pattern in both rotational orientation and in translationalposition.
 5. The method of claim 1 wherein at least one of said beams ofthe second or subsequent exposures of the photosensitive layer is variedin amplitude such that each subsequent interference pattern differs fromsaid first interference pattern.
 6. The method of claim 1 wherein atleast one of said beams of the second or subsequent exposures of thephotosensitive layer is varied in phase such that each subsequentinterference pattern differs from said first interference pattern. 7.The method of claim 1 wherein at least one of said beams of the secondor subsequent exposures of the photosensitive layer is varied in phaseand amplitude such that each subsequent interference pattern differsfrom said first interference pattern.
 8. The method of claim 1 whereinthe periodicity of the interference pattern of at least one said secondor subsequent exposures of the photosensitive layer is varied such thateach subsequent interference pattern differs from said firstinterference pattern.
 9. In microelectronic processing, the method ofproducing a single isolated line of extreme submicron dimensions on aphotosensitive layer comprising the steps of:a) exposing thephotosensitive layer for a first time to two beams of coherent radiationsuch that an image of an interference pattern is formed on said layer;b) isolating a portion of a single line within said interference patternby a second exposure of the photosensitive layer using conventionaloptical lithography.
 10. In microelectronic processing, a method ofproducing interdigitated structures on a photosensitive layer,comprising the steps of:a) exposing a defined area of the photosensitivelayer with a first interference pattern, having a period p1, saiddefined area being bounded by two side edges approximately parallel tothe lines of constant exposure dose and by top and bottom edgesapproximately perpendicular to the lines of constant exposure dose; b)exposing a second defined area containing the top edge of the firstdefined area with a second interference pattern of period p2 equal totwice p1 and with lines of constant exposure parallel to those of thefirst interference pattern, said second interference pattern beingpositioned relative to the first interference pattern such that everyother unexposed region of the first exposure pattern within the seconddefined area is exposed; c) exposing a third defined area containing thebottom edge of the fist defined area with a third interference patternof period p2 equal to twice p1 and with lines of constant exposureparallel to those of the first interference pattern, said thirdinterference pattern being positioned relative to the first and secondinterference patterns such that every other unexposed region of thefirst exposure pattern within the third defined area is exposed, saidunexposed regions being connected to unexposed regions alternate tothose exposed in step b.
 11. The method of claim 10 wherein the secondand third exposures of steps b and c are replaced by a single secondexposure of period p2 equal to twice p1 and with lines of constantexposure parallel to those of the first interference pattern, andfurther, in which both interfering beams of the second exposure passthrough a mask with two transparent holes that map the second exposureinto two areas at the photosensitive layer containing said top edge andsaid bottom edge, respectively, there further being a net phase shift of1/2 period between the two resulting interference patterns at thephotosensitive layer caused by optical path length differences in thetransparent mask areas, said interference patterns being disposed tosimultaneously expose every other unexposed region of said firstexposure within the illuminated areas. .Iadd.
 12. An apparatus forproducing a two-dimensional complex pattern on a photosensitive layer,said pattern containing structures with dimensions in the extremesubmicron range, comprising:a movable table; a wafer positioned on saidmovable table, said wafer having a surface; and, a source of coherentradiation which forms subsequent images of interference patterns on saidsurface of said wafer, said source providing at least two beam paths,said radiation having an amplitude, phase, angle, intensity andperiodicity..Iaddend..Iadd.13. The apparatus of claim 12, wherein saidtable communicates with a means for rotation and a means fortranslation..Iaddend..Iadd.14. The apparatus of claim 12, wherein saidwafer has a photosensitive layer and a substrate..Iaddend..Iadd.15. Theapparatus of claim 12, wherein said source has a means for varying theamplitude of said radiation, a means for varying the phase of saidradiation, a means for varying the angle of said radiation, a means forvarying the optical intensity of said radiation and a means for varyingthe periodicity of said interference pattern..Iaddend..Iadd.16. Theapparatus of claim 12, further comprising a means for dividing saidcoherent radiation into said beam paths, each of said beam paths havingcoherent radiation of essentially equal intensity at said wafer, therebyassuring a high contrast exposure..Iaddend..Iadd.17. The apparatus ofclaim 12, further comprising a phase-amplitude mask, said maskintercepting at least one of said beam paths..Iaddend..Iadd.18. A methodfor producing a two-dimensional complex pattern on a photosensitivelayer, with dimensions in the extreme submicron range, in a stepwisemanner by reducing the field aperture to increase the sourcecoherence..Iaddend.